MIDWEST STATES POOLED FUND PROGRAM

Continued Development and TL-4 Evaluation of Midwest Four-Cable, High-Tension, Median Barrier

Sponsoring Agency Code

TPF-5(193) Supplements 64 and 79

Problem Statement

For the past several years, the Midwest Roadside Safety Facility (MwRSF) has been conducting research for the Midwest States Regional Pooled Fund Program to develop a non-proprietary, high-tension, four-cable, median barrier. Originally, the system was designed for use anywhere in a V-ditch with 4H:1V side slopes. Six full-scale crash tests were conducted in an effort to evaluate this system. These tests evaluated the cable median barrier at various locations in the v-ditch and on level terrain with the 1100C, 2270P, and 1500A vehicles. The full-scale crash test program demonstrated both failures and successes. The information learned from these tests was combined with other crash testing and computer simulation results to develop a revised crash testing matrix for cable median barriers under MASH.

Due to the difficulties that were experienced developing a system that can be placed anywhere in a 4H:1V v-ditch, the members of the Midwest Pooled Fund Program have revised the objective. The modified cable barrier system would be designed for use in median ditches with 1V:6H or flatter side slopes and using a 0 to 4 ft lateral placement away from the edge of the shoulder or slope break point. The system configuration was to be selected so that the barrier would also provide satisfactory safety performance when positioned anywhere in a 1V:6H median ditch. In addition, it was hopeful that the cable barrier system could later be modified for use in 1V:4H sloped medians, including at 0 to 4 ft as well as anywhere in the ditch. Finally, the modified cable barrier system would be designed to meet the Test Level 3 impact safety standards provided in MASH.

MwRSF has been working on redesigning the cable median barrier to meet this new objective while taking into consideration the design issues that were observed in previous full-scale testing. A significant effort was undertaken to improve the performance of the proposed cable median barrier and aid in development of a final test matrix for cable median barriers. This effort absorbed significant funding that was allocated for this project. Thus, a need exists to allocate additional funding for completing the development, testing, and evaluation of the non-proprietary, high-tension, four-cable, median barrier.

Background

After August of 2011, MwRSF set out to redesign the cable median barrier to meet the revised objective while taking into consideration the design issues that were observed in previous testing. Review of the failed crash tests identified three design problems that needed to be addressed to improve the system and meet the new MASH TL-3 test matrix requirements for cable median barrier.

1. The current design of the cable median barrier had difficultly capturing vehicles when the barrier was placed down the slope. Test no. 4CMB-5 showed that impacting the system at a post tended to pull down the top cable and compromise vehicle capture.
2. The current system could cause A-pillar crush. Review of the test no. 4CMBLT-1 suggested that two factors that were contributing to the A-pillar crush were the lateral strength of the post and the release forces of the cable to post attachment.
3. The behavior of the cable-to-post attachments in the current design found that the current attachment behavior was not optimized. The cable-to-post attachments appeared to be too strong vertically and unable to effectively release cables. With respect to the lateral release loads, it was observed that the lateral release forces were not adequately yielding and displacing the upstream and downstream posts in the system to effectively absorb energy. This is critical to maximize the energy dissipation and control deflections.

In order to address these issues, MwRSF proposed a series of design changes to improve the performance of the system. First, redesign of the top cable attachment was proposed to improve vehicle capture. The new design would prevent pull-down of the top cable when the post is impacted and allow for safe vehicle capture. Second, new post sections were investigated with the goal of lowering the lateral or strong-axis strength of the post. It was believed that a post with lower lateral strength would result in lower forces imparted to the A-pillar and reduced A-pillar damage. In addition, lowering the lateral capacity of the post would also allow for easier yielding of the post for energy absorption as compared to the current S3x5.7 post. Data analysis of current cable median barrier systems conducted by Dr. Cody Stolle at MwRSF also suggested that cable median barriers with lower lateral post strengths have reduced tendencies for vehicle rollover. Third, redesign of the cable-to-post attachment was conducted with the goal of lowering vertical release loads and tuning the lateral force to match the new post section. This research and development effort was extensive and consisted of approximately 65 static component tests, 68 dynamic component tests, 23 bogie tests of post and attachment concepts, LS-DYNA computer simulations, and a large amount of engineering analysis.

The redesign effort resulted in the following changes to the system. First, a new Midwest Weak Post design was developed, as shown in Figure 1. This new post section is fabricated from folded sheet steel and provides a reduced lateral strength. In addition, the new post should be less expensive than the S3x5.7 post because it is simple to manufacture and weighs approximately 30 percent less per foot. Second, the cable-to-post attachment for the three lower cables was changed to a tabbed bracket design, as shown in Figure 2. The tabbed bracket design was selected as it allowed for a reduction of the vertical release loads by a factor of more than four while retaining the lateral capacity to yield the post and absorb energy. The tabbed bracket was also believed to be simpler and cheaper to fabricate than the keyway bolt. Third, the top cable-to-post attachment was completely redesigned, as shown in Figure 3, to prevent the pull down of the cable that we observed in the full-scale crash testing. The design of the new top cable connection consisted of a 1.25â€ deep v-shaped slot in the web of the post to provide lateral and vertical support for the cable and a 1/8â€ diameter brass retention rod. Finally, the cable heights of the system were revised to meet the revised objective and the cable installation tension was reduced due to concerns for increased A-pillar damage with high cable tensions.

Based on the projectâ€™s new direction, MwRSF also set out to investigate MASH TL-3 vehicle trajectories in both 6H:1V and 4H:1V V-ditches of various widths while exploring and updating the proposed cable barrier testing matrices that were initially suggested at the May 2011 Summer TRB Workshop and Meeting in Cleveland, Ohio. The draft matrices were prepared based on discussions between MwRSF and TTI and were more robust than those suggested at the April 2011 Midwest Pooled Fund meeting. As a result of the extensive LS-DYNA effort and exploration of trajectories into both ditch slopes of varying width, new cable barrier testing matrices were proposed and documented in a MwRSF-Pooled Fund research report (Report No. TRP-03-265-12, Titled: Test Matrices for Evaluating Cable Median Barriers Placed in V-Ditches).

The proposed updated cable barrier test matrices were then later discussed at the 2012 Joint Summer TRB Workshop and Meeting and AASHTO TCRS meeting in Irvine, California. Modifications to these updated matrices were made by AASHTO TCRS. Based on that vote, AASHTO TCRS in cooperation with MwRSF, TTI, and NCAC personnel are to determine a plan for updating MASH with further refined matrices. A draft of the refined matrices for 6H:1V and 4H:1V ditches are shown in Tables 2 through 5. Note that the refined matrices have to be implemented into MASH and then balloted by AASHTO TCRS and voted upon before they are finalized.

In March 2013, a request for guidance was made to the member states pertaining to the revised system hardware and configuration. The proposed system utilizes bottom and top cable center heights of 13Â½ in. and 38 in., respectively, with the second and third cables spaced evenly in between the top and bottom cables. It was also proposed to reduce the design tension at 100 degrees Fahrenheit to around 2,000 or 2,500 lb. A consensus on this inquiry was not reached. Some member states had no issues with the general hardware and configuration, while others desired more R&D brainstorming on the cable to post connection, adjustment of the cable heights, and discussion of the cable tension.

Objectives

The objective of this research effort is to complete the development of a non-proprietary, high-tension, four-cable, median barrier for use in 6H:1V V-ditches with the barrier located 0 ft to 4 ft laterally away from the slope break point and with consideration for placement anywhere in the ditch. In addition after a successful crash testing program, the cable barrier system should be capable of being adapted for use in 4H:1V V-ditches with a placement of 0 ft to 4 ft laterally away from the slope break point. The system is to meet the TL-3 safety performance criteria found in MASH.

Research Plan

There are two proposed options for the continuation of this research and development project. Option A would consist of accepting the redesigned non-proprietary, high-tension, four-cable, median barrier system and proceeding with full-scale crash testing and evaluation according to the proposed cable median barrier test matrix. Option B would consist of revisiting the cable-to-post connection design with additional brainstorming and R&D to improve its simplicity and ease of installation. Both options are budgeted herein and discussed further below.

OPTION A - Test Current Design (RFPF-14-CABLE-1A)

For Option A, MwRSF is proposing to evaluate the cable median barrier system based on the crash test matrix for anywhere in 6H:1V v-ditch. It is believed that evaluation of the cable median barrier placed anywhere in the 6H:1V v-ditch provides greater installation flexibility and less concern regarding system working width for backside impacts on barriers utilizing a 0-ft to 4-ft offset. Evaluation of the system anywhere in the 6H:1V v-ditch will require eight full-scale crash tests, three 2270P pickup tests, four 1100C small car tests, and one 1500A sedan test, as shown in Table 2. A decision to evaluate the system only for a 0-ft to 4-ft offset in the 6H:1V v-ditch only reduces the test matrix by one test. In addition, if the barrier were crash tested at the 0-ft to 4-ft offset in the 6H:1V v-ditch, four additional crash test would be required for placement anywhere in a 6H:1V v-ditch.

If the Midwest Pooled Fund member states would decide to proceed with evaluation of the cable median barrier located at a 0 ft to 4 ft offset in a 6H:1V V-ditch, the budget could be revised to eliminate a single full-scale crash test.

MwRSF will prepare CAD drawings of the non-proprietary, high-tension cable median barrier system for a 600-ft long system on both level terrain and a 6H:1V v-ditch. MwRSF will construct test systems for five tests in the ditch and three level terrain tests. The non-proprietary, high-tension cable median barrier will be evaluated according to the proposed MASH guidelines for evaluation of cable median barriers anywhere in a 6H:1V v-ditch, as shown in Table 2. The full-scale vehicle crash test will be conducted, documented, and evaluated by MwRSF personnel and in accordance with MASH guidelines.

After completion of the full-scale crash testing, a summary report of the research project will be completed detailing the tested barrier system, full-scale crash test results, evaluation of barrier performance, and recommendations for implementation and barrier system installation. The non-proprietary, high-tension cable median barrier system would also be submitted for approval to FHWA.

OPTION B - Bracket Redesign R&D (RFPF-14-CABLE-1B)

Option B would consist of an effort to brainstorm new ideas for the cable-to-post attachment to make the system easier to install and maintain. This effort would focus on attachments that install with limited tools and that can be installed quickly and easily in adverse environmental conditions. New concepts for the cable-to-post attachment would be developed and submitted to the Midwest Pooled Fund states for review and comment. Concepts that were deemed acceptable by the states would undergo computer simulation using LS-DYNA and dynamic component testing to determine their effectiveness. The results of the research effort would be compiled in a summary report along with recommendations for a new cable-to-post attachment for use in the full-scale testing and evaluation of the cable median barrier system.

Full-scale testing is not budgeted as part of Option B, but it is anticipated that the full-scale testing of the cable median barrier with the revised bracket would follow the plan noted above in Option A in the following Pooled Fund year.

Benefit

The successful completion of the development, testing, and evaluation of the Midwest four-cable, high-tension, median barrier in a V-ditch and on level terrain, will allow the member states to implement a non-proprietary, high-tension, cable system along our nationâ€™s highways and roadways. The crash testing of the four-cable, high-tension, median barrier on level terrain would also provide a more complete understanding of barrier performance (i.e., dynamic deflections, working width, etc.) when used in relatively flat, narrow medians.

Disclaimer:
The information contained on the Midwest Roadside Safety Facility (MwRSF) website is subject to change without prior notice. The University of Nebraska and the MwRSF is not responsible or liable, directly or indirectly, for any damage or loss caused or alleged to be caused by or in connection with the use or misuse of or reliance upon any such content, goods, or services available on this site.